there are so many options i would start with the cheapest option and go from there, some cases offer more protection then others, and some cases will leave some wear on the bezel
I have a $0.35 snap together on its way. I have never liked the harder rubber cases because they never seem to fit the buttons or are so tight that they squeeze the buttons. The soft rubber ones are just floppy crap. I've never even seen a case on a M so I don't know what this will be like
I use the Otter Defender, seems ok, had it about a month now. It fits great BUT the depth of the bezel is kind of a hassle for getting to the "A" and "L" when texting. And, it seems that the phone gets hot quick.
But, I feel safe with dropping it a bit and the holster/clip is pretty stout. I catch it on my seatbelt ALL the time and the phone only drops because of my dumb hand.
I have the Diztronic TPU case on the M. It's perfect for me. Not bulky and protects it well. First drop with case on was ~3ft onto concrete. no damage what so ever. Not even a scratch on the case.
I have and love the Incipio Feather case on my Razr M. It actually provides a better grip on the phone and you hardly know it's there. Couldn't be more pleased.